(1) Field of the Invention
The present invention relates to the field of non-destructive ultrasonic medical and industrial inspections. In particular it relates to ultrasonic color imaging for medical and material inspection and characterization.
(2) Description of Related Art
Taking advantage of the fact that ultrasound can travel through article bodies unreachable by human vision, ultrasonic inspection has been widely employed in industrial material inspection and medical diagnosis for a long time. When a traveling ultrasound signal encounters an acoustical interface dividing two materials of different acoustic impedances, a portion of ultrasound energy enters into the forward material, while the remaining part of signal energy is bounced back to the backward material, generates a detectable ultrasound echo revealing the existence of the interface. Acoustical interfaces capable of generating ultrasound echoes include the exterior surfaces enclosing the target body, as well as the interior discontinuities such as layered structures, voids, cracks, impurities and any abrupt changes in acoustic impedance.
The most popular ultrasonic inspection products can be classified into three major categories: Ultrasonic thickness gauges, Ultrasonic flaw detectors, and Ultrasonic imaging systems.
Ultrasonic thickness gauges measure the time span between two echoes reflected by two interfaces of interest, typically the front and back walls of the target. The thickness, i.e. the distance between the two walls, is readily calculated through the measured time span and the sound speed of the target material. Numerical thickness readings are then displayed on a screen or stored in a data file.
Ultrasonic flaw detectors display a selected portion of received signal trace carrying information about the target interior. Interior discontinuities on the traveling path of incident ultrasound reflect the passing signal, causing extra echoes to appear in the received signal trace. Advanced flaw detectors provide auxiliary tools such as programmable gates, thresholds, cursors, etc, to help locating echoes of predetermined amplitudes within predetermined ranges.
Ultrasonic imaging systems rely on moving an ultrasound beam across the target surface to produce an image of the target interior. The scan of sound beam can be realized either by mechanically moving the probe, or by electronically moving the sound beam formed by a phased transducer array. The produced images are either cross-sectional profiles perpendicular to the scan surface, or a reflecting interface underneath and roughly parallel to the scan surface. The brightness of each image point is determined by the intensity of the echo signal attributable to the corresponding field point within the target.
In all three categories of ultrasonic inspections described above, it is the geometries of the exterior and interior discontinuities that are actually being studied. As a matter of fact, for every field point on an interface, there could be multiple echoes providing information, and each echo supplying a series of numerical data representing signal amplitude as a function of time; In sharp contrast, for all image points between the interfaces (not on the interfaces), there are no data representing/describing them. That is why traditional ultrasonic inspection only inspects interfaces (or discontinuities) within the target.
Many critical physical conditions without distinct geometric boundaries, such as variations in elasticity, density, hardness, stiffness, strength, compositional distribution, metallographic properties, etc. may develop due to diseases, medical treatments, machine processing, material fatigue, uneven strain stresses, temperature gradations, physical strikes, prolonged exposure to physical or chemical effects, etc., are potentially harmful but not in the scope of ultrasonic inspections. It would be highly desirable to detect such conditions before they develop into fatal discontinuities like cracks. Unfortunately, traditional ultrasonic inspections only deal with discontinuities, not continuously distributed conditions.
In traditional ultrasonic imaging, only one parameter, typically the peak amplitude of the first echo, is used as the across board imaging parameter. Field points producing echoes of same peak amplitude are represented with same brightness, therefore seem identical to image viewers. It can not be farther from the truth. In fact, echoes with equal peak amplitudes can be quite different in their waveforms. The waveform difference can reveal critical difference not only about two field points, but also about the paths that the sound traveled to reach each field points. Unfortunately, such invaluable information has been left unused in conventional ultrasonic imaging for decades.
A major improvement was disclosed by the present inventor in patent application “Methods and Apparatus for Ultrasonic Color Imaging”, Application Number 2004100745612 filed with China patent office on Sep. 8, 2004, PCT Application Number PCT/CN2004/001030 on Sep. 8, 2004, and application Ser. No. 11/369,603 filed with USPTO on Mar. 7, 2006, all claiming the benefits of same named provisional patent application filed on Sep. 8, 2003 with China Patent Office.
With the improvement disclosed in these filings, every image point is specified by three color parameters derived from entire echo waveform, not just from the peak of the echo. Field points producing echoes of same peak amplitudes but different waveforms are effectively distinguished by different color compositions. Continuous medium body is presented in a color composition determined by the interfaces confining the medium. However, this improvement is still limited in the way that image points on the same sound path (same scan line) and confined by the same pair of interfaces are represented either identically, or in monotonically changing manner. In reality, the acoustic properties near the interfaces typically vary in a non-linear and non-monotonous manner, the closer to the interface, the more dramatic the acoustic variation is. Prior to the present invention, no effective means of charactering or imaging such acoustic conditions are commercially available.
Some very thin sheet targets, such as the blades of an aircraft engine or power generator operating at extremely high speed, while being most vulnerable to interior defects, are the hardest to inspect by conventional ultrasonic inspections, due to the simple fact that tiny echoes generated by defects are buried in much larger echoes contributed by the front and back exterior walls. Continuous defects that distort rather than reflect sound signals, are by far more harmiftl to thin targets than to thicker targets, but are even harder to detect.
The usefulness of any inspection is largely determined by its visual impacts to the operator. First, visual sense communicates with brain most efficiently. The information that a single glimpse sends to the brain, if to play out in audio frequencies, can take years of listening. In technical language, visual sense possesses a bandwidth significantly broader than other human senses do. Secondly, visual sense has unique spatial perception. Not only the existences of all the objects within the sight, but also the spatial attributes such as dimensions, shapes, relative positioning with each other, can be learned via visual sense almost instantly without training.
In terms of spatial perception and efficiency, visual presentations of all ultrasonic inspections are not equal. The best presentation is the stereoscopic real time images, such as a pumping heart, a breathing lung, or a live baby moving in mother's womb. Not surprisingly, this most desired presentation, known as four dimensional imaging, is produced by very expensive ultrasonic imaging systems.
On the other end, the numerical thickness readings presented by ultrasonic thickness gauges, although efficient in conveying measurement results, provide no spatial perception. A numerical reading is not visually related to the spatial attributes like size or spacing. “0.9999” and “1.0000” look very different but are practically equal, while “0.07” and “0.01” seem more alike but differ drastically. Many thickness gauges use a sound beep to alert the occurrence of pre-defined thickness reading, indicating that numerical readings alone can not fulfill the task satisfactorily. Moreover, in erosion inspection of extended pipelines or gigantic high pressure vessels, hundreds even thousands thickness measurements are needed to locate the worst erosion spots. In such common and demanding inspection tasks, an appropriate image is definitely better than piles of numerical readings.
Signal traces, always used but not necessarily explicitly displayed by all ultrasonic inspection equipments, carry valuable information not shown in numeric readings and traditional images. A false thickness reading, due to severe noises or overlap of multiple echoes, often can be identified from the signal trace. However, users are unable, or to larger extent, unwilling to deal with signal traces because of their poor visual image. It is a demanding task, even for well trained NDT professionals, to identify flaws through tiny echoes buried in much larger interfering echoes and background noises. Whenever possible, most inspectors rather take the straightforward numeric readings over insightful signal traces.
Ultrasonic erosion inspection and flaw detection are rarely performed by imaging systems despite of all the merits of image, not only because imaging systems are prohibitively expensive for typical NDT operation budgets, but also due to the operability limitations of imaging systems. Transducer arrays used in medical imaging can manage an adequate acoustical contact with soft human bodies, but not on rigid, curved surfaces and small facets, and are not practical when the test spots have narrow accesses, high temperature, or other severe conditions. Mechanical scan require good acoustic coupling between the transducer and target without changing target's positioning, which is achieved either by running a stream of coupling agent between the transducer and the target throughout the inspection, or by immersing the scan mechanism in a tank filled with coupling agent (water or oil)—not feasible for most NDT situations.
Above discussions indicate three much needed improvements in ultrasonic inspection: a) inspection or characterization for continuously distributed physical/acoustical conditions; b) inspection or characterization for very thin sheets; c) an inexpensive, practical way to generate visual images in ultrasonic erosion inspection, flaw detection and other common and demanding NDT tasks.